Docs on APIs implemented are inconsistent

[kevinlacaille]

Expected behavior
The docs should say:
Orders
Data (mostly complete)
Subscriptions (in progress)

Then either mention or don't mention all of the other APIs that aren't implemented. Maybe only mention the APIs we plan to implement.

Actual behavior (describe the problem)
The docs on readthedocs, in our README, and on the quick start guide are all inconsistent about what is and isn’t implemented.

[cholmes]

I think this is fixed, with #765 and new docs.